Method for coating inside surfaces of cavities



Dec. 9, 1969 o. L. BUSH ET AL 3,483,097

METHOD FOR COATING INSIDE SURFACES OF CAVITIES Filed Jan. 8, 1968 ATTORNEY United States Patent 3,483,097 METHOD FOR COATING INSIDE SURFACES OF CAVITIES David L. Bush, Englewood, and Robert E. Greb, Miamisburg, Ohio, assignors to General Motors Corporation,

Detroit, Mich., a corporation of Delaware Filed Jan. 8, 1968, Ser. No. 696,457 Int. Cl. C23b /56 U.S. Cl. 204-26 9 Claims ABSTRACT OF THE DISCLOSURE A method for controlling the thickness distribution of electrically deposited coatings on those interior surfaces which define an open-mouthed cavity in a hollow body. The method includes the steps of plugging the mouth of the cavity with an electrically nonconductive insert having an orifice therethrough and subsequently directing plating current through said orifice.

Though our invention is generally applicable to most electrically deposited coatings including both electrolytic and electrophoretic deposits, it is particularly useful in electroplating processes. Heretofore when electroplating the walls of an open-mouthed cavity, such as the cylindrical surfaces of a bore, it has been the practice to employ internal anodes and robber bars as means to effect a uniform thickness of electrodeposit. Absent the use of these expedients, the electrodeposit builds up at the mouth of the bore and very little deposits on the inside thereof.

We have found that we can simply and quickly plate those interior surfaces or walls which define an openmouthed cavity in a hollow body with a uniform thickness of electrodeposit without the use of internal anodes and/ or robber bars. In fact, with our method, the thickness distribution of such electrodeposits over the walls of a cavity may actually be controlled.

It is, therefore, an object of this invention to provide an improved process for controlling the thickness distribution of electrically deposited coatings on the walls of a cavity in a hollow body.

It is a further object of this invention to provide an improved and simplified method for electroplating a uniform thickness of metal on the cylindrical surfaces of a bore.

It is another object of this invention to eliminate the requirement for internal anodes and robber bars as means for controlling the thickness distribution of electroplated deposits on these interior surfaces which define a cavity in a hollow body.

These and other objects of this invention will become more apparent from the detailed discussion which follows.

FIGURE 1 is a partially sectioned view of one embodiment of this invention.

FIGURE 2 is a partially sectioned view of another embodiment of this invention.

FIGURE 3 is a partially sectioned view of still another embodiment of this invention.

This invention relates generally to a method for electrically depositing coatings and more specifically to a method for electrically depositing coatings on those interior surfaces which define an open-mouthed cavity (e.g., a bore) in a hollow body. Our invention is most particularly applicable to a method for electroplating and therefore will be discussed primarily in terms of such a method. Our invention resides in the use of an electrically noncoductive orificed insert member which is positioned so as to plug the mouth of the cavity to be plated. The mouth provides access to the cavity from outside the body. The specific shape of the orificed insert member determines "Ice -' the exact thickness distribution of the electrodeposit. The

insert has one end which is within the cavity and hence inboard of the mouth. The other end is at least flush with, though preferably outboard of, the mouth of the cavity. The shape of the hollow insert is such that it has a varying cross-sectional area when considered in relation to a series of planes which are normal to an axis passing through the respective ends of the insert. Cross-sectional area, as used herein, comprehends the entire area within the periphery of the outside dimensions of the insert. Hence, this area includes both the solid portion of the insert and the void portion of the orifice. The cross-sectional area of the inboard end of the insert is less than that of the outboard end, or at least less than that portion of the insert which is immediately adjacent the mouth of the cavity. The aforesaid cross-sectional area gradually increases from the inboard end toward the outboard end. In the case of a body having a cylindrical cavity therein, that portion of the insert which is inside the cavity may conveniently be a truncated hollow cone or hemisphere. For insert shapes which are based in part on a circle (e.g., cones or hemispheres), a more convenient way of expressing the general character of the inserts shape is by reference to a member which has a varying outside diameter. For such circle-based shapes the outside diameter gets smaller as the inboard end of the insert is approached.

While we do not intend to be 'bound by any theory, the new and novel benefits of our invention appear to result from the ability of the inserts to shield the mouth of the cavity in a manner which prevents the heavy buildup of plate thereat, and to effectively equalize the length of the lines of force between the points on the surface to be plated so that a more uniform current density distribution over the inside surface of the cavity results. The principles taught by our invention apply to all systems for electrically depositing coatings wherein current density variations effect deposit thickness variations. Similarly our invention can be used with most any electroplating bath, though some dimensional adjustments may have to be made to compensate for the different throwing powers of different baths.

FIGURE 1 shows a hollow body 2 having a cavity or bore 4 therein. In this case the surface of the cavity 4 has a substantially uniformly thick electrodeposited coating of metal 20 applied thereto. To effect this, hollow inserts 6 are positioned so as to plug the open ends or mouths 22 of the bore 4. Each hollow insert 6 has inboard and outboard ends 8 and 14 respectively as well as an orifice or throughhole 12 which extends between the ends 8 and 14. In this particular embodiment the external shape of insert 6 is conical and the internal shape of the hole 12 is also conical. The actual shape of the hole 12 is not particularly significant. However, the dimensions of the inboard end opening 10 are quite significant as will be discussed hereinafter in connection with the effects observed when certain dimensions were varied. After insertion of the inserts 6 the part to be plated is immersed in a plating tank 24 such that outboard end 14 of the insert 6 substantially opposes the electroplating anodes 18. An EMF is impressed on the body 2 and anodes 18 by means of a generator G. The body 2 is made cathodic with respect to the anodes 18 and a sufficient EMF is impressed to deposit the coating 20. In the case of an electrophoretic coating the anode or cathode relationship could be reversed depending on the ionic character of the resin deposited.

FIGURE 2. shows a hollow body 2 having a cylindrical cavity or bore -4 therein. To apply a uniformly thick coating 20 to the inside surface of the cavity 4, the hollow inserts 6 are positioned in either end of the bore 4 at the mouths 22 thereof. In this embodiment the 3 external shape of that portion of the insert which is inside the cavity 4 is conical as was that of FIGURE 1. In this embodiment, however, the hollow or bore 12 in the insert 6 is cylindrical rather than conical. Additionally, this embodiment includes a flanged portion 16 which is provided to assist in quickly but accurately centering the insert 6 in the mouth of the cavity. The flange additionally shields the mouth 22 and prevents the buildup of electrodeposited metal thereat.

FIGURE 3 shows another embodiment of our invention which varies from that shown in FIGURE 2 primarily in that the outside shape of that portion of the insert 6 which is inside the cavity 4 is generally hemispherical.

The following is a specific example of our invention and is directed toward a body having a cylindrical cavity or bore therein. In this particular example the cavity has two openings or mouths, one at either end of the bore. The specific example is directed toward a particular conically shaped insert of the type shown in FIGURE" 2 and the particular dimensional relationship associated with that cone to effect a uniform distribution of electrodeposit over the inside surface of a cylindrical cavity. However, it should be noted that we do not intend to limit ourselves to cylindrical cavitie or even to uniform thickness electrocoating. Other deposit thickness distributions may be effected by varying the shape and dimensions of the inserts and the depth of insertion as will be discussed hereinafter. FIGURE 2 shows a hollow body 2 having a cylindrical cavity or bore 4. The outside shape of the body 2 is not significant and hence, for example, it may correspond to the pin end of an internal com bustion engine connecting rod. The bore itself was 1 inch deep (long) and had a finally plated inside diameter of between 0.9898 inch-0.9901 inch. The conically shaped inserts 6 were comprised of polyvinyl chloride and had a Vs inch diameter central bore 12. The bore 12 extended between the inboard end 8 and the outboard end 14 and had an inboard end opening with a diameter of 7 inch. The inserts 6 were positioned in the mouth 22 such that the inboard ends 8 extended 7 inch into the cylindrical bore 4. The conical portion of the insert 6 was such that the angle X formed between it and the plated surface was 60 degrees. The distance between the respective inboard openings 10, after insertion was inch. The surface of the cavity was degreased using trichloroethylene at room temperature and subsequently electrocleaned for about one minute. It was next activated by dipping in an acid solution for about one minute. Next, a 1% minute copper flash from a standard copper cyanide bath was applied to the surface followed by a lead-tin coating which was deposited to a thickness of 0.00030.0004 inch from a standard 88% Pb=12% Sn fluoborate bath. Appropriate water rinsings were used between the principal steps recited above.

When using two opposing inserts for plating the same cylindrical cavity, varying both the dimensions and position of the inserts in relation to the cylinder effects different plating thicknesses inside the cavity. For example, the deeper the inserts are positioned into a cylindrical cavity the more plate is deposited on the center section of the cylinders ID. and the less is deposited around the mouth of the cylinder. Similarly, reducing the depth of insertion will concentrate the plate near the mouth of the cylinder and reduce the plate thickness at the center thereof. Additionally, the larger the opening 10 at the inboard end of the insert, the more plate is deposited near the cavitys mouth 22. Smaller openings 10 tend to direct the plate toward the center of the cylinder. Small angles of taper X deposit more plate near the center of the cylinder and effectively starve the mouth of the cylinder. Larger angles of taper X increase the thickness of plate at the mouth of the cylinder. Clearly, of course, our invention is not restricted to the use of two opposing inserts or to cylindrical cavities. When cavities having other shapes are to be plated or when QII only one insert is to be employed, the shape, dimensions, and positioning of the insert in the mouth will necessarily change to meet the plating thickness requirements of any given situation.

The composition of the insert must be such as to be electrically nonconductive, chemically resistant to the plating environment and related process steps, and relatively resistant to deformation incident to any temperature changes that the plating sequence might have. We prefer to use polyvinylchloride, but many other plastics known to those skilled in the art will also suffice for purposes of practicing our invention.

Our invention is effective with most of the conventional electroplating baths presently commercially available. Each bath, of course, has its own peculiarities, especially with respect to throwing power. These factors should all be considered when selecting a particular shape and size insert to meet the requirements of a given job. We have provided the foundation whereby this selection can generally be made. The precise dimensions will, of course, vary with each requirement. Hence, while we have disclosed our invention primarily in terms of specific embodiments thereof, we do not intend to be limited thereto, except as defined by the claims which foliow.

We claim:

1. A method for controlling the thickness distribution of an electrically deposited coating on the walls of a cavity in a hollow body, which cavity has at least one mouth therein for communicating said cavity with the outside of said body, said method comprising the steps of plugging said mouth with an electrically nonconductive insert having a hole therethrough, said insert having a portion thereof positioned to a predetermined depth within said cavity, said portion being of a shape which will induce a predetermined distribution of an electrically deposited coating material on said walls, said hole having a first end inboard of said mouth and a second end generally outboard of said mouth, immersing said body in a bath of the coating material to be electrically deposited, electrically connecting said body to a counterelectrode, establishing an EMF between said body and said counterelectrode and passing a deposition-inducing current through said hole to deposit said material from said bath onto said walls of said cavity.

2. A method for controlling the thickness distribution of an electrically deposited coating on the walls of a cavity in a hollow body which has at least one mouth therein for communicating said cavity with the outside of said body, said method comprising the steps of plugging said mouth with an electrically nonconductive insert having a hole therethrough, said insert having a portion positioned to a predetermined depth within said cavity, said portion being of a shape such that the cross-sectional area of said portion generally decreases with increasing dis tance away from said mouth into said cavity with the precise dimension of said portion being determined by the coating distribution sought to be effected, said hole having a first end inboard of said mouth, a second end outboard of said mouth and a continuous passage between said ends, immersing said body in a bath of the coating material to be electrically deposited, electrically connecting said body to a counterelectrode, establishing an EMF between said body and said counterelectrode and passing a deposition-inducing current through said hole to deposit said material from said bath onto said walls of said cavity in a predetermined manner.

3. The method as defined in claim 2 wherein said cavity is cylindrical and said mouth is at one end thereof.

4. The method as defined in claim 3 wherein said portion is generally conically shaped.

5. The method as defined in claim 4 wherein said passage is generally cylindrically shaped.

6. The method as defined in claim 2 wherein said in sert has a flange adjacent said mouth of said cavity to facilitate central alignment of said insert in said mouth 5 and to prevent a disproportionately rapid buildup of deposit at said mouth.

7. The method as defined in claim 2 wherein said electrically deposited coating is a metal and said bath is an electroplating bath.

8. The method as defined in claim 7 wherein the precise dimensions of said portion and said predetermined depth of insertion are such as to deposit a metal coating having a substantially uniform thickness over said walls.

9. The method of claim 4 wherein the said thickness distribution is substantially uniform, said cavity is cylindrical and has two mouths, one at either end of said cavity, and each of said mouths is plugged with an insert in the same manner as claimed with respect to the plugging of said one mouth.

References Cited UNITED STATES PATENTS 2,706,175 4/ 1955 Licharz 20426 X 3,376,210 4/1968 Kiefer et al 204-26 X FOREIGN PATENTS 1,432,423 2/1966 France.

429,206 5/ 1935 Great Britain.

DANIEL E. WYMAN, Primary Examiner C. F. DEES, Assistant Examiner 

